The Hephaestus forge needs a structure to be created this structure is a flat 9x9x1 square with the corners missing.

weirdly requires this structure that the corners are air blocks. if these are any other block the structure fails to create the forge.

I don't know if this is intentional but if it is please update the wiki with the the required air blocks/empty space

This issue has been resolved in newer versions. If this still happens for you in 1.19 please comment again and the issue will be reopened.